The present embodiments relate to wireless communications systems and, more particularly, to a multiple space time transmit diversity encoded wireless communication system.
Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”) which includes wideband code division multiple access (“WCDMA”) cellular communications. In CDMA communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. WCDMA includes alternative methods of data transfer, one beoing frequency division duplex (“FDD”) and another being time division duplex (“TDD”), where the uplink and downlink channels are asymmetric for FDD and symmetric for TDD. Another wireless standard involves time division multiple access (“TDMA”) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into slots (i.e., packets) so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a slot in the time period, and that slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to time slots.
Since CDMA and TDMA communications are along wireless media, then the travel of those communications can be affected in many ways, and generally these effects are referred to as the channel effect on the communication. For example, consider a transmitter with a single antenna transmitting to a receiver with a single antenna. The transmitted signal is likely reflected by objects such as the ground, mountains, buildings, and other things that it contacts. In addition, there may be other signals that interfere with the transmitted signal. Thus, when the transmitted communication arrives at the receiver, it has been affected by the channel effect. Consequently, the originally-transmitted data is more difficult to decipher due to the added channel effect. Various approaches have been developed in an effort to reduce or remove the channel effect from the received signal so that the originally-transmitted data is properly recognized. In other words, these approaches endeavor to improve signal-to-noise ratio (“SNR”), thereby improving other data accuracy measures (e.g., bit error rate (“BER”), frame error rate (“FER”), and symbol error rate (“SER”)). Several of these approaches are discussed below.
One approach to improve SNR is referred to in the art as antenna diversity, which refers to using multiple antennas at the transmitter, receiver, or both. For example, in the prior art, a multiple-antenna transmitter is used to transmit the same data on each antenna where the data is manipulated in some manner differently for each antenna. One example of such an approach is space-time transmit diversity (“STTD”). In STTD, a first antenna transmits a block of two input symbols over a corresponding two symbol intervals in a first order while at the same time a second antenna transmits, by way of example, the complex conjugates of the same block of two symbols and wherein those conjugates are output in a reversed order relative to how they are transmitted by the first antenna and the second symbol is a negative value relative to its value as an input. To illustrate this operation, consider the block of two symbols s(0) and s(1) at respective symbol intervals n for (n=0) and (n=1). This block is input to an STUD encoder, which over two symbol intervals outputs those symbols in the following streams 1 and 2 to its respective transmit antennas 1 and 2:
Antenna 1:s(0)s(1) stream 1Antenna 2:−s(1)*s(0)*stream 2Accordingly, there is some redundancy in the sense that a symbol transmitted by one transmit antenna is also transmitted in another form along a different transmit antenna for the same transmitter. The approach of using more than one transmit antenna at the transmitter is termed transmit antenna diversity. Further, note that in the STTD system the transmitted signals are in terms of blocks of symbols (or symbol samples) such as a block of two symbols in the preceding example, as opposed to single sequential symbols. This transmission of signals including such blocks is sometimes referred to as time diversity because each block of symbols represents a period of more than one symbol time. Also, where time diversity is combined with multiple transmit antennas such as in the example of an STTD system, then such a system is sometimes referred to as providing space time encoding.
Another approach to improve SNR combines antenna diversity with the need for higher data rate. Specifically, a multiple-input multiple-output (“MIMO”) system with transmit diversity has been devised, where each transmit antenna transmits a distinct and respective data stream. In other words, in a MIMO system, each transmit antenna transmits symbols that are independent from the symbols transmitted by any other transmit antennas for the transmitter and, thus, there is no redundancy either along a single or with respect to multiple of the transmit antennas. The advantage of a MIMO scheme using distinct and non-redundant streams is that it can achieve higher data rates as compared to a transmit diversity system.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements may be made, including by addressing some of the drawbacks of the prior art. As one example of a drawback, while the STTD system provides transmit antenna diversity, there is still redundancy in that the same symbol is transmitted, albeit in different forms, on each transmit antenna, thereby reducing the effective symbol rate of the transmitter relative to the MIMO system that transmits independent symbols on each of its transmit antennas. However, the STTD system provides orthogonal signals as between the redundant signals on different transmit antennas, and this orthogonality prevents the signals on different antennas from interfering with one another; in contrast, the MIMO system does not provide such orthogonality and, as a result, spatial interference may occur. Indeed, to address some of these issues, the present inventors described a multiple space time encoded system in co-pending U.S. patent application Ser. No. 10/107,275, filed Mar. 26, 2002, entitled, “Space Time Encoded Wireless Communication System”, and hereby incorporated herein by reference. In this referenced application, multiple independent streams of data are connected to corresponding multiple independent space time encoders; each space time encoder then provides, in one embodiment, orthogonal output symbols in response to its input stream, while bandwidth is improved because at the same time an independent stream is also encoded and output by another space time encoder in a comparable manner. Further, a receiver is provided that decodes the transmitted signals including the multipaths therein. While this improvement therefore provides various benefits as discussed in the referenced application, the inventors also recognize still additional benefits that may be achieved with such systems. Accordingly, the preferred embodiments described below are directed toward these benefits as well as improving upon the prior art.